As a hydraulic drive system for controlling a delivery rate of a hydraulic pump depending on a demanded flow rate, there is known the so-called load sensing control (hereinafter referred to as LS control) in which the delivery rate of the hydraulic pump is controlled in response to a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators, as disclosed in JP, B, 60-11706 and JP, A, 1-312201, for example. Such an LS system comprises a variable displacement hydraulic pump, a plurality of actuators connected to the hydraulic pump in parallel and driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of flow control valves provided respectively between the hydraulic pump and the plurality of actuators for controlling respective flow rates of the hydraulic fluid supplied to the actuators, a control lever unit having a plurality of control levers for respectively controlling operation of the plurality of actuators, a pressure sensor for detecting a maximum load pressure among the plurality of actuators, and a pump controller for controlling a delivery pressure of the hydraulic pump to be held higher than the maximum load pressure by a fixed value (i.e., a target LS differential pressure).
When any one of the control levers is operated, the associated flow control valve is opened at an opening corresponding to its input amount or stroke (i.e., a demanded flow rate), whereupon the hydraulic fluid from the hydraulic pump is supplied to the associated hydraulic actuator through that flow control valve. Simultaneously, a load pressure of that hydraulic actuator is detected as the maximum load pressure by the pressure sensor, and the detected maximum load pressure acts on the pump controller which controls a delivery rate of the hydraulic pump so that the pump delivery pressure is held higher than the maximum load pressure by the fixed value. In the above process, when the input amount of the control lever (i.e., the demanded flow rate) is small, the opening of the flow control valve is also small and so is the flow rate of the hydraulic fluid passing through the flow control valve. Therefore, the pump delivery pressure can be held higher than the maximum load pressure by the fixed value with the small pump delivery rate. When the input amount of the control lever (i.e., the demanded flow rate) is increased, the opening of the flow control valve is also increased and so is the flow rate of the hydraulic fluid passing through the flow control valve. Therefore, a larger pump delivery rate is required to hold the pump delivery pressure higher than the maximum load pressure by the fixed value. As a result, the pump delivery rate is increased to maintain the fixed value.
Thus, in the LS control system, the pump controller is operated in response to a differential pressure between the pump delivery pressure and the maximum load pressure (i.e., an LS differential pressure), and the pump delivery rate is controlled depending on the demanded flow rate. Also, because the LS differential pressure is kept constant even with the load pressure of any actuator fluctuating, the differential pressure across the associated flow control valve is also kept constant, whereby the flow rate supplied to that actuator is held at a fixed value corresponding to an opening area of the flow control valve (i.e., the input amount of the control lever). In other words, the actuator is driven at a speed corresponding to the input amount of the control lever without being affected by fluctuations in the load pressure.
The pump controller for the LS control system has been designed with various constructions. Generally, as disclosed in JP, B, 60-11706, the pump controller comprises an adjusting valve operated in response to the LS differential pressure, and an actuator driven by the hydraulic fluid supplied through the adjusting valve for operating a swash plate of the hydraulic pump.
Also, the prior art disclosed in JP, A, 1-312201 adopts a pump controller comprising an unloading valve operated in response to the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure such that it is opened when the differential pressure exceeds a predetermined value for discharging a part of the delivery rate supplied from the hydraulic pump to a reservoir, a resisting device provided downstream of the unloading valve for generating a control pressure corresponding to the flow rate of the hydraulic fluid discharged from the unloading valve, and a negative regulator for reducing the delivery rate of the hydraulic pump as the control pressure generated by the resisting device becomes higher, and increasing the pump delivery rate as the generated control pressure becomes lower. In this pump controller, when the delivery rate of the hydraulic pump is smaller than the demanded flow rate, the pump delivery pressure does not rise so that the differential pressure between the pump delivery pressure and the maximum load pressure, i.e., the LS differential pressure, becomes smaller than the predetermined value, thereby closing the unloading valve. Accordingly, the control pressure generated by the resisting device is lowered and the pump delivery rate is controlled to increase. When the delivery rate of the hydraulic pump is increased above the demanded flow rate, the pump delivery pressure rises so that the LS differential pressure becomes larger than the predetermined value, thereby opening the unloading valve. Accordingly, the control pressure generated by the resisting device is raised and the pump delivery rate is controlled to decrease. Thus, with this prior art, the pump delivery rate is controlled so that the pump delivery pressure is held higher than the maximum load pressure by a fixed value.
Meanwhile, as another type of hydraulic drive system for controlling a delivery rate of a hydraulic pump depending on a demanded flow rate, there is known a control system in which the opening area of a center bypass of a center-open flow control valve is reduced depending on an input amount of the control lever to thereby control the pump delivery rate and the flow rate supplied to the actuator, as disclosed in JP, A, 1-25921, for example. In this case, the actuator is supplied via the flow control valve with the hydraulic fluid at a flow rate resulted by subtracting a bleed rate through the center bypass from the delivery rate of the hydraulic pump. The control effected by this system is called bleed-off control.